1. Field of the Invention
The present invention relates to a caliber roll for rolling consisting a roll main body and a roll shaft, used in caliber rolling of tubes and bars, and a manufacturing method of its roll main body, and more particularly to a caliber roll for rolling possessing a sufficient abrasion resistance and crack resistance characteristic and having an excellent service life, and a manufacturing method of its roll main body.
2. Description of the Related Art
A caliber roll for rolling used in caliber rolling of tubes and bars has, as shown in FIG. 1, a hollow roll main body 1 having a caliber 1a, and a roll shaft 2 tightly fitted into a shaft hole 1b of the roll main body 1. In the case of this caliber roll for rolling, when rolling, a tensile stress .sigma..sub.t acts on the bottom section of the caliber 1a of the roll main body 1, due to the surface pressure P acting on the caliber 1a of the roll main body 1. The distribution of this tensile stress .sigma..sub.t reaches the maximum on the bottom surface of the caliber 1a, and supposing this maximum value to be .sigma..sub.tmax, the surface pressure P is high depending on the rolling condition, and .sigma..sub.tmax rises, and when this .sigma..sub.tmax exceeds the material strength of the roll main body 1, the bottom surface of the caliber 1a is cracked, and thereby the roll main body 1 is broken. Besides, the bottom surface of the caliber 1a of the roll main body 1 is likely to be cracked because it is exposed to cyclic thermal stresses of processing heat and cooling by lubricating oil.
As the countermeasure of roll breakdown, hitherto, the roll material is changed to a stronger material, but the roll cost rises, and generally the higher the strength, the lower becomes the toughness, and cracks due to impact are more likely to occur.
As other method, a gap is provided in the contact surfaces of the roll main body 1 and roll shaft 2 (Japanese Patent Application Publication No. 59-2561, U.S. Pat. No. 4,874,312 (Japanese Patent Application Laid-Open No. 61-216807)). These methods are intended to lessen the tensile stress on the bottom surface of the caliber 1a caused by rolling force, by forming a recess in the middle part of the roll main body 1 or in the corresponding position of the roll shaft 2, and deflecting the roll by the vertical components of the surface pressure while rolling, thereby generating a compressive stress on the bottom surface of the caliber 1a. That is a bending stress is generated in the bottom section of the caliber 1a by rolling reaction, and this bending stress acts as a compressive stress on the bottom surface of the caliber 1a, and by this compressive stress, the tensile stress maximum value .sigma..sub.tmax is reduced, hence preventing breakdown.
However, even by the method of forming a recess in the middle part of the roll main body 1 or in its corresponding position of the roll shaft 2, crack and roll breakdown could not be sufficiently prevented owing to the following reasons.
FIG. 2 shows an example of roll peripheral direction distribution of vertical component (roll reaction) P of surface pressure applied to the caliber roll of cold Pilger rolling mill forming a caliber gradually decreasing in the radius in the peripheral direction, mean tensile stress .sigma..sub.H of caliber bottom section and tensile stress .sigma..sub.T of caliber bottom surface (corresponding to .sigma..sub.tmax in FIG. 1) caused by it, in which the axis of abscissas denotes the position in the roll peripheral direction, and the axis of ordinates is the roll reaction and tensile stress. That is, according to this diagram, the roll reaction P reaches the maximum near section No. 0.3 in the roll peripheral position, the mean tensile stress .sigma..sub.H reaches the maximum nearly at the maximum position of the roll reaction P, and the tensile stress .sigma..sub.T of caliber bottom surface reaches the maximum nearly at section No. 0.55.
The reason of deviation of the maximum position of the tensile stress .sigma..sub.T of caliber bottom surface in the rightward direction or in the caliber radius decreasing direction, with respect to the roll reaction maximum position, is as follows. The mean tensile stress .sigma..sub.H increases as the roll reaction becomes larger, but even at the same roll reaction, as the caliber radius becomes smaller, the two, as shown in the diagram, the maximum position of the tensile stress .sigma..sub.T of caliber bottom surface increases due to stress concentration, and by the effects of the tensile stress .sigma..sub.T of caliber bottom surface is deviated to the caliber radius smaller side. Meanwhile, the multiple breakdown forming region of the caliber roll for rolling in the diagram coincides with the maximum position of the tensile stress .sigma..sub.T of caliber bottom surface.
In the caliber roll for rolling showing such distribution, when the above recess forming technology is applied, the compressive stress generated on the roll caliber bottom surface depends on the roll reaction force itself, and therefore the compressive force generated at the maximum position of the tensile stress of caliber bottom surface is smaller than the compressive stress generated at the maximum position of the roll reaction, and hence the effect by the compressive stress at the maximum position of the tensile stress of caliber bottom surface is small, thereby leading to roll breakage.
The material of the roll main body of the caliber roll for rolling is explained below.
Conventionally, the roll main body of caliber roll for rolling was generally made of SUJ5 steel specified as bearing steel in JIS, or high carbon low alloy tool steel such as 0.8%C-1.75%Cr-0.3%Mo-0.1%V steel (hereinafter the percentage expressing the content of components is wt. %). However, the high carbon low alloy steels are not sufficient in hardening, and large in fluctuations of hardness due to uneven hardening and mass effect, and are likely to cause wear and crack depending on application conditions. For hardening, therefore, instead of hardening the entire section of the roll, a technique called cored hardening for hardening only the surface layer by special heat treatment has been employed. In the roll fabricated by cored hardening, since the hardened portion is only the surface layer, the abrasion resistance is maintained only for a short term, and when the caliber surface layer is worn to a certain extent, the hardness of the caliber surface suddenly drops, thereby leading to collapse of the caliber shape.
Accordingly, as the material of the roll main body, the JIS SKD11 steel (high carbon high alloy tool steel) with excellent hardenability has come to be used. The roll made of this high carbon high alloy tool steel is excellent in hardenability and can be hardened entirely, and special treatment such as cored hardening is not needed. However, the roll main body made of SKD11 steel is required to have a hardness of H.sub.R C 60 or more (Rockwell C scale) from the viewpoint of prevention of caliber abrasion and surface spalling. To endow with such hardness, however, as clear from the tempering temperature curve in FIG. 3, for example, after hardening at 1030.degree. C., tempering must be done at a low temperature of about 200.degree. C. Accordingly, the subsequent heating temperature range is limited, and not only the temperature control is difficult at the time of shrinkage-fitting to the roll shaft, but also softening may be possibly caused by processing heat or abrasion heat in rolling. Furthermore, this SKD11 steel is not sufficient in toughness, and when applied in the roll main body, it is indicated that the caliber is likely to be broken from the bottom during rolling.
In this background it was once proposed to use a cold tool steel (C: 0.75 to 1.75%, Si: 3.0% or less, Mn: 0.1 to 2.0%, P: 0.020% or less, S: 0.003% or less, Cr: 5.0 to 11.0%, Mo: 1.3 to 5.0%, V: 0.1 to 5.0%, N: 0.020% or less, O: 0.0030% or less) with an attempt to enhance the toughness while maintaining the high hardness of the SKD11 steel, on the basis of the SKD11 steel, by decreasing the contents of P, S, O and N, and increasing the content of Mo (Japanese Patent Application Laid-Open No. 64-11945). This steel (hereinafter calls SKD11 modified steel) is superior to SKD11 steel in toughness, realizes the tempering effect by heating at 450.degree. C. or higher, and east in temperature control in shrinkage-fitting, and free from risk of softening due to processing heat during use, but the following problems are known.
That is, the SDK11 modified steel (the cold tool steel disclosed in the Japanese Patent Application Laid-Open No. 64-11945) mainly features the resistance to abrasion by allowing to be used at high hardness by the portion of the superior toughness, and accordingly when applied in the roll main body of the caliber roll for rolling, the appropriate hardness is said to be H.sub.R C 62 to 63. However, if a high impact load is applied as in the caliber roll for rolling, even by application of the SKD11 modified steel, it is difficult to prevent cracks from the caliber bottom, and this tendency is more obvious when used at such high hardness.
Besides, in this SKD11 modified steel, in order to maintain the material hardness of H.sub.R C 62 or 63, the tempering temperature must be 490.degree. to 530.degree. C. in the case of 1030.degree. C. hardening, but as clear from FIG. 3 this is the temperature range before and after the secondary hardening temperature, and even in this temperature range, if exceeding the secondary hardening temperature, the hardness drops suddenly, and such hardness cannot be maintained stably. Therefore, usually, the tempering temperature is below the secondary hardening temperature, and the tensile residual stress of the surface layer (generated as the surface shrinks at the time of cooling when hardening) and residual austenite (expanding by martensiting with the lapse of time) are not eliminated, thereby leaving the factors of cracks.
Thus, in the conventional caliber roll for rolling, the roll wear was excessive, and it was required to adjust the roll gap (adjust the outside diameter) frequently depending on the extent of roll wear, and to prepare the mandrels differing in size (adjust the product wall thickness), and short life of the roll and other problems were not sufficiently solved.